1. Field of the Invention
The present invention relates to, an optical modulator and an optical modulator array. More particularly, the invention relates to an optical modulator and an optical modulator array that modulate an optical signal by utilizing interference between plural rays of guided mode light propagating through multi-mode interference waveguide.
2. Description of the Related Art
Transmission systems that are capable of manipulating the phase and intensity of optical signals are very important. The most common devices among devices for modulating the phase and intensity of optical signals are optical modulators, which have rapidly become key devices. It should be noted that optical attenuators for attenuating the intensity of optical signals are also included in optical modulators in a broad sense. Conventionally used optical modulators include the Pockels cell and the Mach-Zehnder Modulator (MZM).
The Pockels cell is an optical modulator that performs modulation by utilizing the Pockels effect, which is an electro-optic effect. In this device, an electric field is externally applied on an electro-optical crystal disposed between a pair of polarizers, and change in refraction is induced by the applied electric field. The first polarizer splits the input optical signal into linear polarized components having mutually orthogonal vector directions (TE mode and TM mode). When light propagates through the crystal, the TE mode and the TM mode experience different changes in phases due to the applied electric field and only the predetermined polarized component is transmitted through the subsequent polarizer, which is determined by the difference in the induced phase differences between the TE and TM modes. Thus, the intensity of the optical signal is modulated according to the applied electric field.
The Mach-Zehnder Modulator forms a Mach-Zehnder interferometer by using, for example, a Y-branch waveguide, and it is an optical modulator that effects intensity modulation by utilizing an electro-optic effect, and the like. In this device, an optical path of an optical signal is split into two optical paths, and optical signals from these two optical paths are recombined to form an interference fringe. When an electric field is applied on a waveguide arm of each of the optical paths, a phase shift is caused between the optical signals from the two optical paths and the interference pattern is changed. Thus, the intensity of the optical signal is continuously modulated according to the applied electric field.
Among the above-described conventional optical modulators, the Pockels cells are easily fabricated, however, the difference between amounts of phase modulation of the induced TE mode and TM mode causes the overall phase shift to be relatively smaller. Therefore, they usually require a relatively large voltage to be applied on the optical crystal. On the other hand, in the Mach-Zehnder Modulators, it is not necessary to limit the applied voltage, and only the phase modulation of one of the polarization modes is required. Hence, as stated above, the Mach-Zehnder Modulator can provide a larger effect, and therefore is a more preferable choice than the Pockels cell.
Although the Mach-Zehnder Modulators are widely used for the above-described reasons, fabrication conditions for these devices are quite severe, resulting in low production yield. For example, when an inputted optical signal is split into two, ideally, the optical signal must be split precisely in half to obtain a maximum extinction ratio. Splitting of an optical signal is usually achieved by using a Y-branch waveguide or an optical directional coupler.
When the Y-branch waveguide is used, it is necessary to precisely conduct the tip formation of the branch, and use of high-performance steppers, which allow high-resolution photolithography, is required in order to achieve a high yield. As the optical directional coupler, for example, an optical waveguide-type optical coupler is known, wherein two mutually parallel optical waveguides are provided to be partially in close vicinity to each other. However, branching ratios of such optical directional couplers are very sensitive to materials (for example, refractive indices) and production conditions (for example, degrees of channel separations), which make these devices generally non-robust.
For example, with respect to Mach-Zehnder Modulators having a Y-branch waveguide, a number of improvements on the electro-optic effect have been proposed (see U.S. Pat. No. 5,074,631; and K. Noguchi et al., “A broadband Ti: LiNbO3 optical modulator with ridge structure”, IEEE Journal of Lightwave Technol., No. 13, pp. 1164–1168). However, a decrease in an extinction ratio due to uneven optical power splitting is not mentioned in these documents. Further, importance of robustness of the devices is not mentioned in these documents.
Recently, a Mach-Zehnder Modulator, which employs a multi-mode interferometer (MMI) device instead of a Y-branch waveguide in order to achieve even splitting of an optical power, has been proposed (see U.S. Pat. No. 6,618,179). The multi-mode interferometer device has a wide optical waveguide, and utilizes an interference between a plural rays of guided mode light propagating through the waveguide to control light beams. Further, it is also described in this document that N×N optical switches can be formed using N-branch multi-mode interferometer devices.
However in this optical modulator, it is necessary to provide a Y-branch waveguide at the recombination side in order to achieve low loss. That is, the multi-mode interferometer device is used only for splitting an optical power. Further, phase modulation is performed for each of the split optical paths, and therefore, the multi-mode interferometer device is used merely as a passive device, which makes no contribution in increasing an output power. Therefore, the multi-mode interferometer device provides no advantage, other than enabling the even splitting of an optical power, over conventional Mach-Zehnder Modulators.
There are other examples, where a multi-mode interferometer device is used as an optical branching device or an optical coupling device in optical switches (see U.S. Pat. No. 6,643,419; L. W. Cahill et al., “Switching Properties of Generalized Mach-Zehnder Photonic Switches”, Proceedings of CLEO/Pacific Rim 2001, pp. 238–239; and Yen-Juei Lin et al., “Four-channel coarse-wavelength division multiplexing demultiplexer with a modified Mach-Zehnder interferometer configuration on a silicon-on-insulator waveguide”, Appl. Opt., No. 15, pp. 2689–2694). However, in any of these examples, phase modulation is performed for each of the split optical paths and the multi-mode interferometer device is used merely as a passive device. Therefore, conventionally, no optical modulator has been existed, in which a multi-mode interferometer device is employed as an active device involved in phase modulation.
As described above, the currently widely used Mach-Zehnder Modulators, which use a Y-branch waveguide and/or an optical directional coupler, are not capable of evenly splitting an optical power, and have a problem that it is difficult to produce highly precise optical modulators with a high production yield.
On the other hand, Mach-Zehnder Modulators, which employ a multi-mode interferometer device for an optical branching device, or the like, have been proposed in order to achieve even splitting of an optical power. However, in such optical modulators, the multi-mode interferometer device is used merely as a passive device, and therefore, no optical modulator has been existed, in which the multi-mode interferometer device is employed as an active device involved in phase modulation.